Evidently for those who support open borders, some sort of limits of e.g. 400k+ would be necessary to prevent such a result being skewed by infinity being included in the sample.

In that case you just interpret it as: ∞ = ♁ − − 1

That is infinity equals Earth (population) minus Australia (current population, because they’re already here) minus one (me; because if everyone’s coming here then there’ll be some comfy stuff out there and I intend to enjoy it).

I’m not sure if an absolute number makes sense given the capacity of Australia to support people can be increased with technology and thus would change over time.
If you want some evidence based absolute number fine, although I don’t know where you’ll find such a number or calculate one.

Having done a bit of thinking on the topic and the meta-topic, it’s obvious to me that we should attempt to run a PDC working group regarding Aussie population policy next cycle (2018-2019). For all our explanations of why it hasn’t happened yet, OP is right in that we ought to have policy in this space.

Things to consider at a minimum:

natural environmental carrying capacity looking forward over the next 50+ years (could eg CSIRO be commissioned to re-evaluate this every decade?)

built environmental carrying capacity (i.e. if you want more residents you need to fund more (effficient/effective) infrastructure and services)

How (1) & (2) relate to demographic trends (I.E. yes, we can take some net immigration because otherwise we’d decline, assuming we haven’t hit capacity yet)

in many ways as far as the above is concerned a net gain is a net gain, so what proportion of the net immigrants should be voluntary and what proportion refugees?